A. Field of the Invention
The invention relates to printing inks for ink jet printers, and, more particularly, to a solid, hot-melt ink that is suitable for subtractive color ink jet printing.
B. Prior Art
Inks that are to be utilized in document printing by ink jet application are subject to demanding and frequently conflicting limitations.
The first is that imposed by the substrate itself, which is typically a fibrous material (e.g. paper) when viewed on a microscopic level. On such a substrate, ink that is applied to it must not soak in and spread or "bleed" excessively, lest the sharpness of the image be unacceptably reduced. As an example of the degree of sharpness required, current applications require a resolution on the order of 240.times.400 (i.e. 240 dots per inch in the horizontal direction, and 400 dots per inch in the vertical direction) for text printing. High quality graphics printing may require resolutions extending up to 600 dots per inch. Further, the ink must be resistant to abrasion, as well as to inadvertant transfer to adjacent materials. Additionally, since the document may be carried through a variety of climates, the ink must remain stable at temperatures ranging from--20.degree. C. to +70.degree. C. These restrictions are fairly severe when applied to black ink, but are even more severe when applied to colored inks, since even minor changes in color are frequently noticeable.
In the case of inks that are to be applied by ink jet techniques, the problem is even further compounded by the requirement that the inks be compatible with jetting requirements of the printer. Typically, this application imposes restrictions on, for example, the viscosity of the inks, so that the inks can be expelled from the jet as relatively uniform droplets without excessive satelliting. Furthermore, the inks must be stable at operating temperatures as high as 150.degree. C. or even 175.degree. C. Additionally, the constituents must all be compatible with each other, both physically (i.e. capable of forming a solution which can be jetted successfully) and chemically (i.e. not adversely affecting the stability or other characteristics of the materials).
Most of the inks heretofore utilized in ink jet printers have been aqueous solutions of water-soluble dyes. Examples of such inks are described in U.S. Pat. No. 3,846,141, issued Nov. 5, 1974 to D. G. Ostergren et al, and U.S. Pat. No. 4,512,807 issued Apr. 23, 1985 to A. Ogawa et al. Inks of this type have been found acceptable for many applications, but do not have the desired hardness of the resultant print, nor the requisite stability and sharpness for fine color applications.
An example of a non-aqueous ink jet ink is described in U.S. Pat. No. 4,390,369, issued June 28, 1983 to A. R. Merritt et al. The ink described therein utilizes a natural wax which is liquefied by heating in order to jet it and which is thereafter solidified on contact with the substrate, e.g. paper. Among other drawbacks, however, the resultant material readily abrades and is thus unacceptable for most document purposes where some degree of print stability is required.